fix(app): prevent scrollend position rollback

Discard virtualizer measurement corrections suppressed during an active user gesture instead of replaying stale offsets at scrollend. Preserve post-scroll live anchoring and add unit plus Electron coverage for ordinary scrolling without updates.

app/src/renderer/src/session-timeline-scroll-policy.mjs

@@ -1,13 +1,8 @@
 export function createSessionTimelineScrollPolicy({ isUserScrolling, writeScroll }) {
   let explicitDepth = 0;
-  let deferredAdjustment = 0;
-  let deferredInstance = null;
 
   function scrollToFn(offset, options = {}, instance) {
     if (explicitDepth === 0 && isUserScrolling()) {
-      const adjustment = Number(options.adjustments);
-      if (Number.isFinite(adjustment)) deferredAdjustment += adjustment;
-      deferredInstance = instance;
       return;
     }
     writeScroll(offset, options, instance);
@@ -22,19 +17,8 @@ export function createSessionTimelineScrollPolicy({ isUserScrolling, writeScroll
     }
   }
 
-  function flushDeferredAdjustment(instance = deferredInstance) {
-    if (isUserScrolling() || deferredAdjustment === 0 || !instance?.scrollElement) return false;
-    const adjustment = deferredAdjustment;
-    deferredAdjustment = 0;
-    deferredInstance = null;
-    const offset = Number(instance.scrollElement.scrollTop) || 0;
-    writeScroll(offset, { behavior: 'auto', adjustments: adjustment }, instance);
-    return true;
-  }
-
   return {
     scrollToFn,
     runExplicit,
-    flushDeferredAdjustment,
   };
 }

app/src/renderer/src/session-timeline-viewport.mjs

@@ -133,10 +133,6 @@ export function useSessionTimelineViewport({
     tailFollowReady.value = true;
   }
 
-  function settleUserScroll() {
-    return scrollPolicy.flushDeferredAdjustment(virtualizer.value);
-  }
-
   return {
     virtualRows,
     totalSize,
@@ -145,7 +141,6 @@ export function useSessionTimelineViewport({
     scrollToIndex,
     scrollToEnd,
     isFollowingTail,
-    settleUserScroll,
     resetForInitialSnapshot,
     completeInitialSnapshot,
   };

app/src/renderer/src/views/SessionDetail.vue

@@ -63,7 +63,6 @@ const liveReloadCoordinator = createSessionLiveReloadCoordinator({
 });
 
 function handleUserScrollEnd() {
-  timelineViewport.settleUserScroll();
   if (active.value) void liveReloadCoordinator.flush();
 }
 

app/tests/electron-session-virtualization.mjs

@@ -323,6 +323,45 @@ async function run() {
   assert(disclosure.unmounted, 'the expanded tool row unmounts outside overscan');
   assert(disclosure.restored, 'disclosure state survives unmount and remount');
 
+  const passiveScrollSettlement = await win.webContents.executeJavaScript(`new Promise(resolve => {
+    const wrap = document.querySelector('.detail-wrap');
+    const originalScrollTo = wrap.scrollTo.bind(wrap);
+    const blockAutomaticScrollEnd = event => event.stopImmediatePropagation();
+    let phase = 'scrolling';
+    let postScrollEndWrites = 0;
+    wrap.addEventListener('scrollend', blockAutomaticScrollEnd, true);
+    wrap.scrollTo = (...args) => {
+      if (phase === 'settled') postScrollEndWrites++;
+      return originalScrollTo(...args);
+    };
+    wrap.dispatchEvent(new WheelEvent('wheel', { deltaY: 70, bubbles: true }));
+    const startedAt = performance.now();
+    function frame(now) {
+      wrap.scrollTop += 55;
+      if (now - startedAt < 600) {
+        requestAnimationFrame(frame);
+        return;
+      }
+      const beforeScrollEnd = wrap.scrollTop;
+      phase = 'settled';
+      wrap.removeEventListener('scrollend', blockAutomaticScrollEnd, true);
+      wrap.dispatchEvent(new Event('scrollend'));
+      setTimeout(() => {
+        const afterScrollEnd = wrap.scrollTop;
+        wrap.scrollTo = originalScrollTo;
+        resolve({ beforeScrollEnd, afterScrollEnd, postScrollEndWrites });
+      }, 120);
+    }
+    requestAnimationFrame(frame);
+  })`, true);
+  assert(
+    passiveScrollSettlement.postScrollEndWrites === 0
+      && Math.abs(passiveScrollSettlement.afterScrollEnd - passiveScrollSettlement.beforeScrollEnd) < 1,
+    `ordinary scrolling settles without rollback (${JSON.stringify(passiveScrollSettlement)})`,
+  );
+  await win.webContents.executeJavaScript(`document.querySelector('button[title="First"]')?.click()`, true);
+  await delay(350);
+
   await win.webContents.executeJavaScript(`window.location.hash = '#/sessions'`, true);
   await waitFor(win.webContents, `!document.querySelector('.virtual-timeline')`, 'session detail deactivation');
   await win.webContents.executeJavaScript(`(async () => {

tests/session-timeline-scroll-policy.test.mjs

@@ -3,7 +3,7 @@ import assert from 'node:assert/strict';
 
 import { createSessionTimelineScrollPolicy } from '../app/src/renderer/src/session-timeline-scroll-policy.mjs';
 
-test('virtualizer scroll writes are deferred throughout a user scroll', () => {
+test('virtualizer scroll writes are discarded throughout a user scroll', () => {
   let scrolling = true;
   const element = { scrollTop: 100 };
   const writes = [];
@@ -21,12 +21,13 @@ test('virtualizer scroll writes are deferred throughout a user scroll', () => {
   assert.deepEqual(writes, [], 'momentum is never interrupted by a programmatic write');
 
   scrolling = false;
-  policy.flushDeferredAdjustment(instance);
-  assert.deepEqual(writes, [{ offset: 100, behavior: 'auto', adjustments: 24 }]);
-  assert.equal(element.scrollTop, 124, 'the accumulated correction restores the reader anchor once');
-
-  policy.flushDeferredAdjustment(instance);
-  assert.equal(writes.length, 1, 'settlement is idempotent');
+  assert.equal(element.scrollTop, 100, 'scrollend never replays a suppressed correction');
+  policy.scrollToFn(100, { behavior: 'auto', adjustments: 8 }, instance);
+  assert.deepEqual(
+    writes,
+    [{ offset: 100, behavior: 'auto', adjustments: 8 }],
+    'new corrections produced after scrollend remain available for live commits',
+  );
 });
 
 test('explicit UUID and pagination navigation can bypass the user-scroll guard', () => {
@@ -44,3 +45,24 @@ test('explicit UUID and pagination navigation can bypass the user-scroll guard',
 
   assert.deepEqual(writes, [{ offset: 720, behavior: 'auto' }]);
 });
+
+test('settlement never replays a measurement correction from before the latest user scroll', () => {
+  let scrolling = true;
+  const element = { scrollTop: 100 };
+  const writes = [];
+  const instance = { scrollElement: element };
+  const policy = createSessionTimelineScrollPolicy({
+    isUserScrolling: () => scrolling,
+    writeScroll: (offset, options) => {
+      writes.push({ offset, ...options });
+      element.scrollTop = offset + (options.adjustments || 0);
+    },
+  });
+
+  policy.scrollToFn(100, { behavior: 'auto', adjustments: -24 }, instance);
+  element.scrollTop = 300;
+  scrolling = false;
+
+  assert.deepEqual(writes, [], 'the completed gesture owns its final scroll position');
+  assert.equal(element.scrollTop, 300, 'scrollend must not roll the viewport backward');
+});

tests/session-timeline-virtualization.test.mjs

@@ -49,7 +49,7 @@ test('timeline viewport owns measurement and anchoring while SessionDetail alone
   assert.match(viewportModule, /scrollToFn:\s*scrollPolicy\.scrollToFn/);
   assert.match(viewportModule, /useScrollendEvent:\s*true/);
   assert.match(viewportModule, /isScrollingResetDelay:\s*450/);
-  assert.match(viewportModule, /settleUserScroll/);
+  assert.doesNotMatch(viewportModule + sessionDetail, /settleUserScroll|flushDeferredAdjustment/);
   assert.match(viewportModule, /useAnimationFrameWithResizeObserver:\s*true/);
   assert.match(viewportModule, /scrollPaddingEnd/);
   assert.match(viewportModule, /scrollToIndex/);